The present disclosure relates to an antenna device and communication device which have a plurality of antennae to provide a diversity effect or the like, and are used by being incorporated in an apparatus including a notebook computer and a mobile terminal.
In the recent trend of using networks, notebook computers, mobile terminals, such as cellular phones and smart phones, game machines, and so forth are provided with a wireless communication capability. At present, most of those types of the apparatuses are provided with, for example, the IEEE 802.11 wireless LAN (Local Area Network) capability. It is also desirable to provide a wireless communication capability which covers a metropolitan area, such as WiMAX (Wireless Metropolitan Area Network).
A monopole antenna is known as the fundamental structure of antennae used in radio communications. The monopole antenna has a monopole with a length which is a quarter of the wavelength λ; the monopole is mounted on a conductive bottom board. To house an antenna into an apparatus, it is necessary to make the antenna compact. An inverted F antenna is known as such an antenna. The inverted F antenna has the distal end of the pole bent to become shorter, and has a short-circuited portion provided near the power supply node to achieve impedance matching.
It is also known that using a plurality of antennae provides the diversity effect. For example, there is a proposal on a computer device having a plurality of inverted F antenna buried in a unit (see, for example, Japanese Patent Application Laid-Open No. 2006-325178).
In case of a notebook computer with a clam shell structure in which a cover including a display panel is mounted on the computer body including a keyboard in an openable and closable manner, the antennae are preferably disposed at the distal end of the cover to provide high reception sensitivity. To provide the diversity effect, it is preferable to set the distance between two antennae as long as possible (see FIG. 19).
Since the inverted F antenna can be designed compact and short, this antenna can be laid out as shown in FIG. 19. However, the inverted F antenna undesirably has a narrow fractional bandwidth. The inverted F antenna is widely used in the IEEE 802.11 wireless LAN in the band of 2.4 to 2.5 GHz (see, for example, Japanese Patent Application Laid-Open No. 2009-44604), but has a difficulty in covering up to the 2.5 to 2.7 GHz WiMAX.
FIG. 20 exemplifies the S11 characteristics (return loss or reflection coefficient) of an inverted F antenna according to a related art. The normal specifications on the S11 characteristics expect that the S11 parameter is less than −10 dB. When the inverted F antenna is used in the bandwidth of 2.4 to 2.7 GHz which is the bandwidth of the WiMAX added to the bandwidth of the wireless LAN, the edge of the frequency illustrated in FIG. 20 is the limit.
Those two wireless communication capabilities can be achieved by providing another antenna for the 2.5 to 2.7 GHz WiMAX in addition to the antenna for the 2.4 to 2.5 GHz wireless LAN. However, the use of 2-system antenna increases the antenna mounting area. In addition, the increase in the number of antennae increases the connections to a signal processing circuit, thus making the internal configuration of the antennae-mounted apparatus complex.
In short, while antennae are preferably separated as far as possible to provide the good diversity effect (it is ideal to dispose antennae at the horizontal ends of the cover in the example illustrated in FIG. 19), this layout becomes the bottleneck in designing the apparatus and interferes with the compact design thereof.